A related-art air-conditioning system including a ventilator includes a refrigerant circuit including a compressor, a four-way valve, an outdoor heat exchanger, an expansion unit, and an indoor heat exchanger. The refrigerant circuit is filled with refrigerant. During cooling, the refrigerant is compressed into high-temperature high-pressure gas refrigerant by the compressor and is then delivered to the outdoor heat exchanger. The refrigerant which has flowed into the outdoor heat exchanger radiates heat to air and thus liquefies.
The liquefied refrigerant is subjected to pressure reduction into a two-phase gas-liquid state by the expansion unit. In the indoor heat exchanger, the refrigerant removes heat from ambient air and thus gasifies. On the other hand, the air from which heat has been removed can cool an indoor space. The gasified refrigerant returns to the compressor.
The ventilator performs an operation of exchanging indoor air for outdoor fresh air. A high enthalpy of air introduced from outdoor air during cooling causes a cooling load (outdoor air load). Examples of other loads include a load (indoor load) generated in an indoor space and a load (overall heat transfer load) due to heat entering through a wall of a structure.
It is necessary for air-conditioning devices to process the outdoor air load, the indoor load, and the overall heat transfer load. The outdoor air load and the indoor load include a latent heat load as well as a sensible heat load. Related-art air-conditioning systems have processed the latent heat load while keeping a temperature (refrigerant evaporating temperature) in the indoor heat exchanger at a constant low temperature.
In such an operation of processing the latent heat load while the evaporating temperature is kept at a constant low temperature, however, operation efficiency (energy efficiency) is reduced because the operation is performed while the low evaporating temperature, which leads to a large power consumption, is kept regardless of load. Increasing the evaporating temperature can reduce the power consumption, thus improving the operation efficiency. Disadvantageously, however, latent heat will be processed insufficiently, causing an increase in humidity of indoor air. This may result in a reduction in comfort.
An air-conditioning system has recently been developed which sets an evaporating temperature upper limit determined on the basis of the humidity of indoor air and controls an evaporating temperature within a range at or below the set evaporating temperature upper limit on the basis of the evaporating temperature, the temperature and humidity of indoor air, and a set temperature while avoiding insufficient processing of latent heat (see, for example, Patent Literature 1).